Antenna arrays are commonly used for transmitting and receiving RF (Radio Frequency) signals in mobile communication systems and are, in such communication, normally dedicated to a single frequency band or sometimes two or more frequency bands. Single frequency band antennas have been used for a long time and normally include a number of antenna elements arranged in a vertical column. A second column of antenna elements needs to be added next to the first column if a network operator decides to add another frequency band using single frequency band antennas.
Due to the rather substantial space requirements of single band columns of antenna elements, and since such an arrangement may be sensitive to interference between the RF signals in the different frequency bands, dual band antennas (or multiple band antennas, such as triband antennas) have been disclosed. One such prior art arrangement 10 is schematically disclosed in FIG. 1. Two types of antenna elements 11, 12 are arranged alternatively in a column, and aligned along a symmetry axis. A first antenna element 11 is a dual band antenna element which operates in two different frequency bands FB1 and FB2 using first 11′ and second 11″ elements, respectively. A second antenna element 12 is an antenna element, which operates in only one frequency band FB2. Although this solution has the drawback that the frequency bands FB1 and FB2 will couple to each other due to the closeness of the parts making up the antenna element, space savings often compensate for these drawbacks. Due to the said drawbacks, however, this kind of configuration is most suitable when the frequency bands are widely separated, for example when the centre frequency of FB2 is approximately twice the centre frequency of FB1.
This kind of dual band antennas, however, are useful when an antenna arrangement is to be used for azimuth control. Such an antenna arrangement matrix 20 is disclosed in FIG. 2. The arrangement 20 comprises two parallel dual band columns 21, 23 of the kind described in FIG. 1. Between said columns 21, 23 is arranged a column 22, parallel to the columns 21, 23, and having single band elements operating in said second frequency band FB2. As is obvious, the antenna arrangement 20 may include any number of columns, every second being of the kind 21, 23 and every second of the kind 22. Using an antenna arrangement as disclosed in FIG. 2, the azimuth angle of a radiated beam may be controlled by imposing a phase shift to a common signal fed to said columns, said phase shift generally being different for each one of the columns, and also for each operating frequency FB1, FB2 (i.e., the azimuth angles of the lobes of the beams radiated by the elements operating in said first frequency band FB1 and said second frequency band FB2, respectively, may be individually controlled). Moreover, these differences can be adjusted by means of adjustable phase shifting means. Preferably, the phase angle difference between adjacent columns of elements will always be mutually the same in order to obtain a wave front substantially in the form of a straight line, wherein the azimuth angle of this wave front can be adjusted by adjusting said phase shifting means.
A problem with the device disclosed in FIG. 2, however, is that it may impose an ambiguity as regarding the direction of arrival (DoA) of a received signal.
Consequently, there exists a need for an antenna arrangement that is able to operate in two or more spaced apart frequency bands, and that is able to determine a correct azimuth angle of received transmissions.